1. Field of the Invention
The present invention relates to a reference track detection system for a disc storage unit, such as a fixed disc storage unit, and more particularly, to a reference track detection system for detecting a reference track based on which a read/write head is then displaced and positioned at each of a number of tracks defined on one or both surfaces of a hard disc, so that the read/write head reads and writes information to and from each track.
2. Description of the Prior Art
In a disc storage unit, as is well known to those skilled in the art, a read/write head must be correctly positioned at a specific track before information or data is read out or written into a specific track on the surface of a disc. The displacement of the head is carried out by driving an actuator. In order to properly position the head at a specific track, it is necessary that the actuator be instructed as to which direction and to what extent the head must be displaced. To this end, for instance, track numbers, which are generally called cylinder numbers, are sequentially assigned to all the data tracks on one surface of the disc. In response to the difference in cylinder number between a track at which the head is positioned at present and a specific track to which the head is to be positioned subsequently, a direction in which the head must be displaced and an amount e.g. the number of tracks by which the head must be displaced, are computed. The disc storage unit always stores therein the cylinder number of the track at which the head is positioned at present. After the head is displaced to the desired position, a new cylinder number is stored into the disc storage unit while the old cylinder number is erased.
When the disc storage unit is de-energized, however, the data of the present position of the head is destroyed. In addition, the head position is not definite when the disc storage unit is energized again, so that the stored data of the head position must be set to a correct value. In order to set the stored data at such a correct value, one specific track of a number of tracks defined on one surface of a disc, is defined as a reference track. At any required time, such as the time that the disc storage unit is started, the head is first displaced to the reference track, and then the stored data representing the present position of the head is initialized to the cylinder number assigned to the reference track. A specific track of a number of data tracks (in most cases, the No. 0 track on the outermost radius) is defined as the reference track. Alternatively, a track adjacent to the data track, and independent of the data track, is defined as a reference track.
However, when the head is displaced to the reference track in order to initialize the above-mentioned stored data of the cylinder number, it is necessary that the proper position of the head at the reference track be detected by suitable means. For this purpose, a head position sensor is incorporated into the disc storage unit. As the head position sensor, a so-called photo-interrupter has been used in general. As is well known to those skilled in the art, the position sensor detects the position of an object in response to variations of interrupted conditions of an extremely narrow light beam when an object moves across the light beam. The photo-interrupter is incorporated into the disc storage unit in such a way that the photo-interrupter can directly detect the position of the head, can detect the position of the head through a movable carriage upon which the head is mounted, or through a small member attached to the carriage.
The position sensor, and especially the photo-interrupt of the type described above used for the detection of the reference track, can detect whether or not the head is properly positioned on the reference track with a high degree of accuracy by sufficiently narrowing the cross section of the light beam, but has the drawbacks that such an accurate photo-interrupter is very expensive, and it takes a long time to adjust and install the position sensor in the disc storage unit.
With respect to a disc storage unit, there has always been an increasing demand for making the disc storage unit more compact in size, and for increasing its storage capacity. In order to meet such needs, there is no choice but to decrease the distance between adjacent tracks as much as possible, thereby increasing the storage capacity. It follows, therefore, that the position sensor is required to have high accuracy, on the order of a few microns or more. As a result, the cost of a position sensor and its associated intergrated circuit becomes inevitably high. Furthermore, since the disc storage unit must be more compact in size, there also arises the problem how to find space for the installation of the position sensor. Moreover, in the case of a highly accurate position sensor, the position sensor must be assembled with a high degree of precision. To this end, extremely fine adjustment of the position of the position detector within a narrow space must be accomplished, and accordingly adjustment becomes extremely difficult, resulting in increased cost for installation and position adjustment of the position sensor in excess of the price of the position sensor and its associated intergrated circuit. In addition, with the increase of an accuracy of the position sensor, there arises a further problem in that the operation of the position sensor is likely to become sensitive to vibration, oscillation and impact.
The above-described fundamental problems cannot be solved as long as a position sensor such as the photo-interrupter is used to detect the reference track.